Evaluation of Central Auditory Processing of Azeri-Persian Bilinguals Using Dichotic Listening Tasks in First and Second Languages.

Objectives
Sometimes there is no hearing impairment, but it is possible to have an auditory disorder. This is known as a central auditory processing disorder (CAPD). Speech dichotic tasks are useful tools to evaluate CAPD, but there is almost no tool to assess this for Azeri people in their native language. The aim of this study was to evaluate central auditory processing of Azeri participants by Azeri dichotic digit test (ADDT).


Materials & Methods
Participants were 52 normal Iranian Azeri students (mean age 23.27± 4.71; 26 females, 26 males) in the Department of Audiology, School of Rehabilitation Sciences, Iran University of Medical Science, Tehran, Iran in 2016. They were chosen by convenient sampling. ADDT was constructed and administered in free recall conditions along with a Persian dichotic digit test (PDDT). After two to four weeks, reliability was performed.


Results
The mean of the right ear score of PDDT and ADDT was 98.90% and 99.09%, respectively. ADDT was reliable in almost all scores. There was no significant difference in performance between men and women in any score of both tests (P>0.05). The results showed a significant difference between both ears' scores in PDDT (P<0.02) as well as in ADDT (P=0.00). The right ear advantage was seen in both tests.


Conclusion
All participants performed significantly better on digits presented in the right ear than the left ear in both tests. Central auditory processing of Azeri participants for Azeri is similar to that for Persian.


Introduction
Central auditory processing (CAP) is the perceptual processing of auditory signals in the central nervous system (CNS) and the neurobiological mechanisms which are substraiting of electrophysiological auditory potentials. CAP encompass sound localization and lateralization; sound discrimination; auditory pattern recognition; temporal characteristics of auditory signals. Central Auditory Processing Disorder (CAPD) is problems with the perceptual processing of auditory signal in the CNS, which reflects by deficiency in one or more of the above mentioned central auditory behaviors (1). Children with CAPD have normal hearing sensitivity and intelligence, but they have some problems with perception of speech in hard listening tasks. The minimum prevalence of CAPD is about 3% to 7% of the population (2), while a prevalence of about 20% has been reported for the students in the United State schools (3). However, children with CAPD typically have no evidence of neurological disease and the diagnosis is made on the basis of performance on behavioral auditory tests (4). Verbal materials used in the tests of CAPD (5).
Dichotic listening procedure was introduced in 1954, in which the competing pairs of digit series are presented simultaneously into the two ears (6). The following studies, conducted using this procedure, demonstrated its potency in exploring both normal and pathologic function of the brain by auditory stimuli in adults. Dichotic digit test (DDT) includes series of most familiar digits presented to both ears at the same time (7).
Normal right-handed individuals obtain greater magnitude of the right ear score compared to the left ear score by using linguistic stimuli in dichotic listening. This phenomenon is called the Right Ear Advantage (REA). The REA for verbal stimulus described in this way that right and left ears are connected to the left and right brain hemispheres, respectively. The contralateral pathways make these connections. Left brain hemisphere is dominant for language processing. The contralateral pathways are more premier than the ipsilateral pathways which organize the link between the ear and the hemisphere on the same side. These explanations of the ear advantages in dichotic listening has been known as the structural hypothesis (8,9).
DDT is a fast and simple test, developed to evaluate central auditory processing system in dichotic conditions (10,11) in both adults and children. This test is sensitive to various central auditory system defects such as; brainstem lesions, cortical and sub-cortical disorders (11), and interhemispheric abnormalities. Sensitivity and specificity of DDT are high (10,11). DDT is not affected by mild to moderate high-frequency loss (12). DDT is described by three scores, including the right ear score, the left ear score, and the right ear advantage (11). Clinical interpretation of dichotic tests depends on REA score. Several studies have used DDT, as a screening tool or part of a test battery, for identification of central auditory processing disorders (7,8,10). Large asymmetry between the two ears may be a sign of a processing disorder usually linked to the left ear as a left-ear deficit (13). For this purpose, some training examinations named "Dichotic listening therapy" were performed (14), and were used for rehabilitation of some disorders like learning disorders, dyslexia, and autism (15).
A study was conducted at the development and standardization of Persian double PDDT on 81 normal Persian adults aged 18 to 32 in Iran and found that DDT was a useful tool to assess the Evaluation of Central Auditory Processing of Azeri-Persian Bilinguals Using Dichotic... auditory processing system in Persian adults (16 (11,18).
Randomized dichotic digits test has been adapted for using in the Persian language and its test-retest reliability and inter-list equivalency was confirmed (19).
Today, the number of bilingual speakers is more than monolingual speakers in the world's population (20). The first language or mother tongue is called L1 and the second language or foreign language is called L2 (21). With regard to language acquisition, bilinguals may be classified as simultaneous, early and late (21). Bilingualism is a widespread phenomenon in Iran and is not limited to a certain area. About 24% of Iran's population speaks Azeri (22).
DDT is available in different forms, including single, double and triple (23). The Persian versions of single DDT (24), double DDT (16,17,25) and randomized DDT are available (18,19) and they are widely used in clinical and research purpose (26)(27)(28)(29)(30)(31), but there is no possibility of examination of central auditory system in Azeri people by DDT with their native language. It is necessary to assess central auditory processing of children as soon as possible in childhood. Therefore, it is required to have access to screening tests of Azeri language.
Adults were selected as the participants of the present study because the development of dichotic tests is done first in adults (7,13,18,19,23), and the results are generalized between ages of 12 toward adolescence. The development of dichotic tests in children is fundamental to find its basic norm values in aged 7 to 11 yr (12,17,24,25).
In this study, double Azeri Dichotic Digit Test

Materials & Methods
This cross-sectional comparative study was conducted with feasibility and reliability measuring tools which included two main steps of construction and administration.

Construction
The Persian stimuli in digit level are currently available and include twenty unique double sets of nine 1-10 monosyllabic digits (16,17). Since the monosyllabic digits below 10 in Azeri language are limited, we used three digits above 10, consisting Hz was recorded on a CD with intensity equal to the average intensity of digits (7,16,32).
Twenty-seven different items were made, where each item included four digits. Pair digits were recorded on two separate channels on a CD. In this way, regardless of two training items, the final test was made of 25 items (100 digits) and the total score for each ear was 100%.

Ethics consideration
All participants were selected from Azeri students of Iran University of Medical Sciences (IUMS) and Tehran University of Medical Sciences (TUMS) in Tehran, Iran.
All participants completed informed consent form.
The ethical aspect of this study was approved by the Research Ethics Committee of IUMS in letter 94/1809/105/d.

Inclusion criteria
An Azeri participant was considered as one born in East or West Azerbaijan or Ardabil Provinces of Iran, acquired Azeri language first, but the Persian language (L2) learn relatively early in childhood before the age of five to seven years old and had no interaction with other languages; no history of otologic, neurologic, and audiology disorders, bilateral normal hearing and normal word recognition score; threshold asymmetry less than participants were asked to repeat all the digits of both ears in any order (10). Using practice items before the main test ensured the examiner weather participants understood the test. Participants responded to the test orally and each test lasted 4 min. Both tests were carried out on participants randomly to eliminate the sequence effect. Scores of the right ear, the left ear and the difference between two scores (REA) of PDDT and ADDT were obtained. After two to four weeks the re-test was conducted on 33 participants with the same conditions. All tests were done by main researcher.

Statistical Analysis
The Kolmogorov-Smirnov test was used for comparing the data distribution against the normal distribution. Spearman correlation coefficient was used to assess the reliability. We Utilized paired t-test to compare the two test scores between the test and the re-test. Independent t-test was used to investigate the effect of gender on the results of each test. We used paired t-test to compare both test scores between both ears to evaluate the ear effect.
The data were processed in SPSS 21 (Chicago, IL, USA) at the significance level of 0.05.

Results
Participants were 52 Azeri students ( Test-retest reliability was surveyed by comparing the mean scores of the first and the second run. All scores of both tests were reliable (P>0.05) with the exception of the left ear score of ADDT (P=0.02). To investigate the reliability of both tests, Spearman correlation coefficient was also used. Total scores in bilinguals were repeatable (P<0.05), but the right ear score of ADDT and the REA score of PDDT were not reproducible ( Table   2). The analysis of the data for determining the effect of gender on results can be seen in Table   3. In both tests, there is no significant difference in performance between men and women in any score of PDDT and ADDT ( Table 3)

Discussion
This study was aimed to construct and assess the ADDT and PDDT. REAs exist in both Persian and Azeri stimuli in bilingual speakers, although the values of REA were not the same (Figure 1).
The mean of the right ear, the left ear and REA scores for PDDT in the present study (98.9, 98.03 and 0.82) are consistent with another result (17).

Reliability
The reliability was assessed using correlation coefficients and paired t-test. Total scores based on Spearman correlation coefficient were repeatable but the right ear score of ADDT and REA score of PDDT were not reproducible. In this way, four out of six calculated scores showed repeatable results.
Results are reliable (Table 2). There was no learning effect on any score of any tests except the left ear score of ADDT (Table 2).
Left ear performance for Azeri stimuli was different from that of the right ear. In the present study, we consider the interval of two to four weeks between the first and second run like previous study (17).
This interval between two run tests was probably not appropriate for bilinguals.

Gender effect
In the present study, the gender did not affect the results of PDDT and ADDT. This finding is in agreement with other studies (35)(36)(37)(38) in which both sexes showed analogous performance. However, there is a disharmony with earlier studies that reported a significant sex difference in the magnitude of laterality effects (39).

ADDT vs. PDDT
The results of the present study clearly indicated This study is replicated in children for tracking maturation trends in the ear scores for the first language and the second language.
In conclusion, Azeri version of double DDT is a reliable tool in almost all scores. Gender has no effect on any score of ADDT. Although the values of REA for PDDT and ADDT were not the same, REAs were observed for both ADDT and PDDT.
Using ADDT to assess central auditory processing of Azeri people in dichotic listening seems to be useful. Ali Akbar Tahaei and Jamileh Fatahi wrote the manuscript with input from all authors and also approval of the version to be published.

Acknowledgement
All authors agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.